1. Technical Field
The present invention relates to a silicon-alloyed cast steel material which is more preferably suitable for piston rings and cylinder liners. The invention moreover relates to piston rings and cylinder liners which comprise such a steel material as main casting. The invention furthermore relates to a method for the manufacture of a silicon-alloyed cast steel material.
2. Related Art
Piston rings seal the gap that exists between the piston head and the cylinder wall against the combustion chamber. During the up and down movement of the piston, the piston ring on the one hand slides with its outer circumferential face in continuously resilient contact against the cylinder wall, on the other hand the piston ring, due to the tilting movements of the piston, slides in its piston ring slot in an oscillating manner, wherein its flanks alternately contact the upper or lower slot flank of the piston ring slot. With the sliding partners each of which runs against the other, more or less severe wear occurs as a function of the material, which wear upon dry operation can lead to so-called seizures, scoring and finally destruction of the engine. In order to improve the sliding and wear behaviour of piston rings relative to the cylinder wall the latter was provided with coatings of various materials on its circumferential surface.
To manufacture parts of internal combustion engines subjected to high loads such as for example piston rings, cast iron materials or cast iron alloys are mostly used. In engines subject to high loads, piston rings, more preferably, compression rings, are subjected to increasing loads among these compression peak pressure, combustion temperature, EGR, lubricating film reduction which decisively influence their operational characteristics such as wear, firing resistance, microwelding and corrosion resistance.
DE 3717297 for example discloses a piston ring of cast iron as sole material with cast iron white-solidified only in a region in its outer circumferential surface caused through exposing the cast iron material to radiation of high energy density and with thermally loaded intermediate region formed between the cast iron base metal and white solidified region.
EP 0821 073 discloses a cast iron alloy with pearlitic basic structure and spherical and vermicular graphite precipitations which, because of the strength values which are also resistant at high temperatures, can more preferably be used for application in piston rings.
The cast iron materials according to the prior art however have a high fracture risk so that when using current materials ring fractures frequently occur. Increased mechanical-dynamic loads result in shorter life spans of piston rings or cylinder liners. Intensive wear and corrosion on running surface and flank likewise occurs.
Higher ignition pressures, reduced emissions as well as direct fuel injection mean increasing loads for piston rings. The consequence is damage and plating-on of piston material mainly on the lower piston ring flank.
Because of the higher mechanical and dynamic loads of piston rings ever more engine manufacturers demand piston rings of high-quality steel (tempered and highly alloyed such as for example material 1.4112). Iron materials with less than 2.08% by weight of carbon are designated as steel. If the carbon content is higher, it is called cast iron. Steel materials have better strength and toughness properties since there is no interference through free graphite in the basic structure.
High chromium-alloyed martensitic steels are mostly used for producing steel piston rings. However, using these steels has the disadvantage that the manufacturing costs compared with cast iron components are significantly higher. Since the steel is purchased as wire (analytically defined material) from external suppliers at relatively great expense less added value is achieved.
Steel piston rings are manufactured from profile wire. The supplied profile wire is wound round, cut open and pulled over an “out-of-round” mandrel. On this mandrel the piston ring receives its desired out-off-round shape through a heat-treatment process, as a result of which the required tangential forces are set. A further disadvantage of the manufacture of piston rings from steel is that from a certain diameter the ring manufacture (winding) from steel wire is no longer possible. In contrast, piston rings of cast iron are already cast out-off-round so that they have an ideal shape from the start.
Further advantages of this manufacturing method of steel piston rings are the dependency on the suppliers (since there are only a few) and the inflexibility in terms of material changes and chemical composition.
Cast iron has a substantially lower melting temperature than steel. Depending on the chemical composition the difference can amount to as much as 350° C. Cast iron is thus easier to melt and cast since a lower melting temperature means a lower casting temperature and thus less contraction due to cooling, as a result of which the cast material comprises fewer blow holes as well as heat and cold cracks. A lower casting temperature furthermore results in less load on the mould material (erosion, gas porosities, sand inclusions) and the oven as well as lower melting costs.